Researchers from North Carolina State University recently conducted a study, published in the scientific journal PLOS One, to determine how the critters could be dangerous even after successful exterminations.

The researchers examined apartment complexes in Raleigh that were infested with bed bugs. They tested the indoor dust that was visibly infected by bed bugs and the indoor dust that wasn’t. A professional pest company then successfully exterminated the infestations, and the scientists inspected the spaces a second time.

They suspected that the insects left behind a compound in their feces called histamine. Although histamines help the immune system fight off germs and repair injuries for humans, they can also have damaging effects, including rashes from physical contact and respiratory issues from inhalation.

After analyzing the results, they found that “histamine levels in bed bug infested homes were at least 20 times higher than histamine levels in homes without bed bugs,” coauthor Zachary DeVries said in a statement.

“A combination of heat treatment to eradicate bed bugs and rigorous cleaning to eliminate some of the household dust could be a way to reduce these histamine levels,” DeVries said. “We’ll do future testing to bear that out.”